Re: mdadm / RAID, a few questions

["Mathias Burén" <mathias.buren@gmail.com>]

On 29 October 2010 21:22, John Robinson <john.robinson@anonymous.org.uk> wrote:
> On 29/10/2010 15:18, Mathias Burén wrote:
>>
>> Hi,
>>
>> I've a few questions in relation to mdadm and performance. System
>> details follow below:
>>
>> Intel Atom 330 @ 1.6Ghz (dualcore, HT), 4GB RAM
>
> [...]
>>
>> Question 1: I saw that in Linux 2.6.36 (perhaps earlier versions as
>> well) you have the kernel config option CONFIG_MULTICORE_RAID456. I
>> tried enabling it, booted to 2.6.36 from 2.6.35, and rebuilding of the
>> array continued where it left off before reboot. However, the
>> performance was abysmal.. around 16MB/s compared to 70MB/s without the
>> option turned on. Is this a bug, or is it because the Atom has no
>> grunt to speak of?
>
> No, the performance of MULTICORE_RAID456 is abysmal on any CPU. It's an
> experimental implementation that doesn't work terribly well. If you're
> interested in developing, by all means help, but if not, turn it off.
>
>> Question 2: The array is now recovering since I've grown it to 6 from 4
>> devices:
>> $ cat /proc/mdstat
>> Personalities : [raid6] [raid5] [raid4]
>> md0 : active raid5 sdf1[0] sde1[5] sdg1[6] sdc1[3] sdd1[4] sdb1[1]
>>       5851054080 blocks super 1.2 level 5, 128k chunk, algorithm 2
>> [6/6] [UUUUUU]
>>       [===========>.........]  reshape = 57.7% (1126387328/1950351360)
>> finish=718.0min speed=19125K/sec
>>
>> unused devices:<none>
>>
>> Is there a way to speed it up? /proc/sys/dev/raid/speed_limit_min is
>> 100000 (100k), /proc/sys/dev/raid/speed_limit_max is 1000000 (1000k).
>
> No, that's probably about right on something as weak as an Atom. Let it run.
>
>> Question 3: Before I created this RAID5 array I did a quick RAID0 test
>> array just for fun, using 2 full devices (not partitions). Now I have
>> this:
>
> [...]
>
> Sorry, I don't know the answer to this. I suspect it's to do with superblock
> versions, but I don't know - I'm sorry that's not helpful.
>
> Cheers,
>
> John.
>
>

Hi,

Thanks for the answers. I got the speed up to around 36 MBps by
disabling NCQ and changing some cache values, so there's only 36
minutes left or so now. I would love to help develop the
multicore_raid456, but I'm not a coder. Patches welcome though!

// Mathias
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